A conventional radar radiates a pulse or a sequence of pulses of duration T and observes any returned signals arriving after the time T. This implies that there is a dead range D=cT/2 (where "c" is the velocity of propagation of the pulses in the medium) around the radar in which nothing can be observed. This dead range is usually not of much consequence. For instance, for T=1 .mu.s we get D=150 m, which is much too short a range to be of concern if flying airplanes are observed. For a taxi radar one wants a shorter dead range, say 15 m, but in this case one wants shorter pulses too in order to get a better range resolution, and the dead range becomes unimportant. The situation is very different for probing radars that have to penetrate highly absorbing media like wet soil, water with dissolved minerals, or hot or molten rock. Very long pulses are required to penetrate such materials and get returned signals with sufficient energy. The dead range becomes a serious problem in this case.
Conventional radar obtains the distance of a target by measuring the round-trip time of an electromagnetic pulse reflected or backscattered by a target. The time diagram of FIG. 1 shows the radiated pulse or pulse sequence 10. Usually this pulse or pulse sequence is used to modulate a sinusoidal carrier, but this modulation is of no interest here and is actually avoided in ultra-wideband radar. Also shown is a returned pulse 12. Neither time nor amplitude are drawn to scale in FIG. 1; the amplitude of the returned pulse is many orders of magnitude less than the amplitude of the radiated pulse and the time .DELTA.T at which the returned pulse is received is usually at least one order of magnitude larger than the pulse duration T.
During the interval of radiation 0&lt;t&lt;T the receiver input is blocked by the radiated signal. Some time is needed for the receiver to recover from this saturation blockage, which creates the dead zone 0&lt;t&lt;T' shown in FIG. 1, and the resulting observable range T'.ltoreq.t.ltoreq.T.sub.max, where T.sub.max is the maximum range for the system. In terms of distance, the dead zone has the length D=cT'/2, where c is the velocity of light, c=1/.sqroot..epsilon..mu., in a loss-free medium, .epsilon. being the permittivity of the medium and .mu. being the permeability of the medium.
In a medium with large losses, long pulses or long sequences of pulses must be used to obtain a return signal without having to radiate a pulse with unrealistically large energy. What happens in this case is shown by FIG. 2. A rectangular pulse 14 with amplitude E.sub.0 and duration T=71.4 ms is radiated through sea water. (This example is chosen because it was calculated in considerable detail in the doctoral thesis of R. N. Boules, "Propagation Velocity of Electromagnetic Signals in Lossy Media in the Presence of Noise", Department of Electrical Engineering, Catholic University of America, Washington, D.C. 20064). If this signal propagates 1.1 km, it is distorted to the pulse 16 with peak amplitude of about 0.5E.sub.0 shown in FIG. 2. If the receiver cannot operate during the time 0&lt;t&lt;71.4 ms, only the part of the distorted pulse for t&gt;71.4 ms is available for use. But this part contains only about half the energy of the whole distorted pulse, as is quite evident from FIG. 2. In order to make use of all of the energy of the distorted pulse, one must find a way to receive it during the time 0&lt;t&lt;T, with T=71.4 ms in FIG. 2, when the transmitter is radiating.
Accordingly, it is an object of the invention to provide an improved technique for probing absorbing media.